晚全新世至今北台灣高山湖泊沉積物特性及降雨型態關聯探討

Abstract

東亞季風區為全球季風帶中一個重要的子系統，其範圍影響全球近四分之一的人口。台灣位於副熱帶東亞季風區，其歷史及地質紀錄可提供西太平洋副熱帶地區季風及颱風變化。然而，藉由台灣湖泊沉積物所重建之晚全新世氣候變化在不同的湖泊紀錄中出現分歧。造成此種分歧的可能原因之一為缺乏現代湖泊沉積物觀測的紀錄，使得控制湖泊沉積物特性變化的機制不明確，進而增加了進行古氣候解釋的不確定性。 為了探討此議題，本研究使用沉積物蒐集器於台灣東北部翠峰湖(24°30’ N, 121°36’ E, 海拔1840公尺)自2017年7月起進行月份湖泊沉積物的觀測。觀測期間橫跨冬季季風影響月份、夏季季風影響月份，以及颱風降雨事件。本研究亦於湖泊最深處鑽取岩心，以重建此區域古環境的變化。本研究使用多重指標對月份湖泊沉積物以及岩心進行分析，包含粒徑分析、總有機物化學分析(總有機碳含量、總氮含量)、穩定碳同位素分析、以及正烷烴分析。岩心亦進行X射線螢光光譜分析(XRF)、鉛210定年、及碳14定年。本研究採集及分析湖泊集水區範圍內之植物及沉積物樣本，以探討湖泊沉積物可能的來源。 月份蒐集的湖泊沉積物分析顯示沉積物顆粒在高水位的時期變細，此期間與冬季季風吹拂的季節相符。在岩心中，平均沉積物粒徑大小變化指示台灣東北部自3000年至今的氣候狀況相對穩定。但藉由端成分分析沉積物粒徑，可發現在此時間段，岩心中粗顆粒端成分的比例逐漸降低，這指示了東北台灣所記錄到的東亞冬季季風在此期間逐漸增強。此時間段東亞冬季季風的增強或可與西伯利亞海自4500年至今的海冰擴張連結。 在岩心中發現三層具低有機質含量的白色黏土層，可能與山崩後底岩裸露風化的紀錄有關。在岩心頂部亦可發現森林砍伐以及森林大火的紀錄。將岩心中事件層紀錄移除後，翠峰湖的湖泊沉積物岩心紀錄可反應古氣候變化。 本研究結果展現台灣東北部高山湖泊紀錄可東亞冬季季風變化。本研究亦展示了影響湖泊沉積特性的不同機制，例如山崩、森林大火、以及人為開發所造成的影響。這指出在進行使用湖泊沉積物進行古氣候解釋前，充分了解比對現代湖泊觀測紀錄、歷史紀錄、以及岩心紀錄的重要性。

Monsoon rainfall in East Asia is an essential sub-system of the global monsoon system and affects nearly one-quarter of the world’s population. Taiwan, an island situated in the subtropical East Asian monsoon system, provides the unique opportunity to study monsoon and typhoon variability over the western subtropical Pacific on both historical and geological time scale. However, the Holocene paleoclimate records retrieved from various lakes in Taiwan have shown discrepancies in their recorded climate variability. This could partly be caused by the lack of modern sedimentological observations, which limits our understanding of the mechanisms controlling lake sedimentation and thus introduces uncertainty in the interpretation of lake records. In order to tackle this issue, sediment traps were collected monthly since July 2017 at Cueifong Lake (24°30’ N, 121°36’ E, 1840 m above sea level) in northeastern Taiwan. The collected data encompasses winter and summer monsoon seasons, as well as typhoon events. A sediment core retrieved from the deepest part of the lake was also taken to reconstruct environmental variations in the catchment. A multi-proxy approach was applied for both the monthly collected sediment and the sediment core, including grain-size analysis, bulk organic (TOC, TN) analysis, stable carbon isotope (13C) analysis, and biomarker analysis. XRF core scanning, 210Pb dating, and 14C dating were also applied to the sediment core. Plant and sediment samples from the vicinity of the lake catchment were collected and analyzed to assess the possible sources of lake sediment. The records from monthly collected sediment traps indicate that a decrease in sediment grain size occurred during sustained high lake level periods, which typically are found during the winter monsoon season. The mean grain size in the core record indicates that the climate in northeastern Taiwan was relatively stable over the past 3 kyrs. However, the coarse grain population extracted by end-member modeling analysis depicts a decrease in content since ~1100 yr BP, indicating a gradual strengthening of the East Asian Winter monsoon (EAWM) in northeastern Taiwan. The enhanced EAWM can be tentatively linked to the expansion of sea ice cover in the East Siberian Seas since 4.5 kyr BP. Three whitish clay layers characterized by low organic content were found in the core sediment, implying the possible cause to be landslides exposing deep-weathered regolith to erosion. The impact of deforestation and forest fire were also recorded in the topmost part of the sediment core. By removing these event-like layers, a paleoclimatic record in Cueifong lake could be obtained. The result in Cueifong Lake clearly demonstrates the unique ability of alpine lakes in northeastern Taiwan to act as truthful recorders of EAWM variability. This study also demonstrates how multiple factors, such as landslide, forest fire, and human activities, can affect lake sedimentation. This highlights the need to better link modern observations, historical records, and paleo-records before performing paleoclimate interpretations based on lake sediments.